Ferroelectric semiconductors combining ferroelectricity with semiconductors remain elusive because most present-day ferroelectric materials are unsuitable for such a combination. Herein we show first-principles evidence towards the realization of a new class of 2D ferroelectric/multiferroic semiconductors that possess high mobility, modest bandgaps, and distinct ferroelectricity that can be exploited for developing various heterostructural devices with desired functionalities. For example, we propose applications of the 2D chemical functionalized materials as 2D ferroelectric field-effect transistors with ultrahigh on/off ratio, topological transistors with Dirac fermions switchable between holes and electrons, ferroelectric junctions with ultrahigh electro-resistance, and multiferroic junctions for controlling spin by electric fields. All these heterostructural devices take advantage of the combination of high-mobility semiconductors with fast writing and non-destructive reading capability of non-volatile memory, thereby holding great potential for future multifunctional devices.[1] Nano Lett., 2016, 16, 7309; 2016, 16, 3226; 2017, 17 6309

Vertically (off-plane) ferroelectric ordering in ultra-thin films has been pursued for decades. We report the existence of intrinsic vertical polarization orderings in CuInP2Se6, a member of layered metal chalcogen-diphosphates (MCDs). Using first-principles calculations and the electrostatic-energy model, we find that the ground state of bulk CuInP2Se6 is ferroelectric (FE) while that of monolayer is antiferroelectric (AFE), and the critical thickness for this FE/AFE transition is around 6 layers. On the other hand, because of the small energy difference but the large barrier between FE and AFE orderings, the FE state can be practically realized in monolayer, giving rise to two-dimensional ferroelectrics. Applying Monte Carlo simulations, we further predict the corresponding ferroelectric Curie temperature (Tc) and electric hysteresis. Our results not only help understand recent contradictory measurements of MCDs but also shed light on realizing and tuning vertical 2D polarization.

The hybrid two-dimension electron gas (2DEG)/ferroelectric (FE) heterostructure system has many promising applications, including field-effect transistors and non-volatile memory. Such devices exploit electrostatic gating, utilizing the high local electric field produced by the FE substrate to shift the Fermi level in the 2DEG. Thus, controlling magnitude and direction of polarization of the FE can significantly alter conductivity and majority charge carrier in the 2DEG. However, fabrication of real devices results in an imperfect interface, degraded by adsorbates and poor surface roughness of the underlying ferroelectric substrate. The study aims to elucidate the dependence of interfacial interaction between 2DEG’s (i.e. transition metal dichalcogenides and graphene) and low-coercivity, high-remnant-polarization perovskite ferroelectrics on surface roughness and fabrication methodology. Characterization was completed with atomic force microscopy, Raman spectroscopy, X-ray photoemission spectroscopy (XPS) and transmission electron microscopy (TEM).

In this paper, we present a systematic study of a series of 1-D magnonic crystals defined as periodic arrays of grooves in a Permalloy thin film. Thin films of Ni80Fe20 with dimensions 350 μm by 10 μm and thickness 40 nm were prepared on a top of the signal line of coplanar waveguides by a combination of photolithography, e-beam lithography, and lift-off processing. Starting from thin films, stripe-type periodic structures (grooves) with a depth of 4 nm were created in a controlled manner by focused ion beam. By changing the dimensions (width) of the grooves, while keeping the distance between such consecutive grooves constant, it was possible to systematically tune single, double, triple, and quadruple ferromagnetic resonance (FMR) absorption modes. Broadband FMR experiments were carried out to collect the frequency and field-dependent FMR spectra. The experimental results are corroborated with micromagnetic simulations.

For manganite superlattices, it has been known that the interfacial effect can give rise to strikingly new physical properties. In this work, we investigate the influence of the superlattice periodicity on electronic phase separation (EPS) phenomena in manganites. We combine magnetic force microscopy (MFM) studies and model calculations to study the n-dependence of EPS of the (La5/8Ca3/8MnO3)2n/(Pr5/8Ca3/8MnO3)n superlattices, and found that the EPS length scale depends sensitively on n. Magnetic and transport measurements also indicate that the physical properties change non-monotonically with increasing n, reaching a minimum at n ~ 6 or 7 for both the Curie temperature and the metal-insulator transition temperature. With increasing n over 7, the physical properties of the superlattice are governed by those of the La5/8Ca3/8MnO3 and the Pr5/8Ca3/8MnO3 layers.

In the last few years, doped metal oxides, including hafnium oxide (HfO2) and zirconium oxide (ZrO2), were found to possess a ferroelectric phase. As compared to traditional ferroelectric perovskites, ferroelectric HfO2 has the advantages of a high coercive field, excellent scalability, and good compatibility with CMOS processing. In this project, we systematically studied the material properties of ferroelectric HfO2 doped with aluminum. We fabricated metal/Al-doped HfO2/Si capacitors with various compositions, top metal electrodes, annealing temperatures and durations. Comprehensive characterizations were carried out including positive-up-negative-down (PUND) measurements, hysteresis, C-Vs, I-Vs, retention, and endurance. We find that the annealing temperature and duration play a critical role in the ferroelectricity of the HfO2 film. The remnant polarization is also influenced by the ratio of hafnium to aluminum. The optimized process conditions result in remnant polarization up to ~10 µC/cm2 for 20 nm Al-doped HfO2 with ±10 V program/erase voltages. This new ferroelectric material will enable a series of novel nanoelectronic and photonic devices.

We will report on thin films of host-guest organic inclusion compounds using a hexagonal Tris (o-phenylenedioxy) cyclotriphosphoazene (TPP) host and included molecular dipoles. The goal is a ferroelectric phase driven by dipole-dipole interactions. We need dipole-dipole interaction energies which exceed kBT as well as small rotational barriers. We have grown films of TPP with xylenes (xylene@TPP) and pyridazine (pyridazine@TPP) as inclusion compounds and are investigating more complicated pyridazine-based molecules in inclusion compounds with TPP. X-ray diffraction on films of xylene@TPP and pyridazine@TPP show hexagonal structure, to have a preferred (0001) orientation normal to the substrate, and hexagonal in-plane lattice spacing of about 11.6Å and c-axis spacing of about 9.9Å, consistent with x-ray powder patterns of similar inclusions. Optical ellipsometry has shown the films to be flat with an optical index of 1.5-1.6. Films co-deposited on interdigital capacitors allow us to measure dielectric properties of the dipole system. Dielectric spectroscopy suggests a dielectric constant at 300K of 3 for monoclinic TPP, and 6-7 for xylenes@TPP and pyridazine@TPP. Dielectric loss peaks are observed near 100K for these simple inclusion compounds.

In this talk, we will first present a discovery of a 2D ferroelectric
materials family, rooted in III2-VI3 van der Waals materials and distinctly
characterized by exhibiting versatile ferroelectricity with both in-plane
and out-of-plane electric polarization. The device potentials of these new
2D ferroelectric materials will be demonstrated using the van der Waals
heterostructures. Based on these discovered 2D ferroelectric materials, we
further propose a design of 2D multiferroic materials via doping a small
amount of magnetic ions into the 2D ferroelectric material. This design
provides a novel multiferroic system based on van der Waals 2D materials,
and these 2D multiferroic materials are expected to have strong coupling
between the ferroelectric polarization and the magnetic order.

The energy transduction between electromagnetic waves and elastic waves plays a key role in modern information technology. Due to the much smaller speed of sound than speed of light in solids, the scattering, diffraction, and localization of GHz acoustic phonons all take place in the mesoscopic length scale, which is challenging for spatially resolved studies. In this work, we demonstrate the imaging of electroacoustic energy transduction in ferroelectric domains by microwave impedance microscopy. Finite-element modeling is used to simulate the excitation of elastic waves and the dissipation of electrical power. In sharp contrast to the standing-wave patterns due to wave reflection from hard boundaries, the interference-like fringes with one-wavelength periodicity are the consequence of sign reversal of the piezoelectric coefficients in opposite domains. Combining the local imaging of energy transduction and numerical simulation, our approach may open up a new area to spatially resolve the field distribution in phonon-polariton systems.

Iron and bismuth are known to be immiscible and unable to chemically bond with each other. Recent success in use of high-pressure techniques to synthesize FeBi2 under high pressure and temperature has inspired many to ask what physical properties it would possess. In this spirit, we have prepared Fe/Bi superlattice by e-beam evaporation to form thin film heterostructures with alternating Fe/Bi bilayers of various thicknesses and individual layer thickness ratios. The interfaces are expected to be smooth and sharp because of the immiscibility, though such simple expectation may have overlooked the surface or interface energy factors that would bear consequences on the uncertainty of crystallographic anisotropy. In this work, the bismuth and iron layers are grown with different sequences or on different substrates or substrate orientations. The physical properties are investigated in order to understand the coupling effects of the heterostructures in correlation to material structures. First-principle calculations were conducted to assist the data interpretations from the perspective of electronic structures.

Recent advances in use of high-pressure techniques have enabled a US team to successfully create the first man-made iron-bismuth binary compound, i.e., FeBi2, at high pressure and temperature. This inspired us to ask whether an Fe/Bi superlattice of proper thickness ratios can exhibit similar physical properties. We thus fabricated by e-beam evaporation the superlattice thin film heterostrcutures of various alternating Fe/Bi bilayer thicknesses. As Fe and Bi are immiscible, the interfaces are expected to be reasonably smooth, though such simple expectation faces uncertainty of crystallographic anisotropy. Some orientations of the bismuth layers would grow more favorably with Fe layers of certain orientations. First-principle calculations were conducted to understand the basic physics underlying structural stabilities and associated electronic structures. The interfaces between the rhombohedral Bi(001) and bcc Fe(001) faces exhibits Moirée pattern from the well-defined periodicity of coincidence-site lattices. The physical properties are understood from systematic comparisons of the bilayer thickness ratios and individual thicknesses per se in accordance both with the theoretical and experimental findings.